On a fresh Xcode install (including some CI provider configurations), there is
no pre-existing testing set that can be used to identify simulator models. Use
the default device set to detect available models instead. Live testing
simulators are still created in the testing set.
Add a lock to ensure that only one iOS testbed per user can start at a time, so
that the simulator discovery process doesn't collide between instances.
Uses symlinks to install iOS framework into testbed clone, adds a verbose mode
to the iOS runner to hide most Xcode output, adds another mechanism to disable
terminal colors, and ensures that stdout is flushed after every write.
Adds a `use_system_log` config item to enable stdout/stderr redirection for
Apple platforms. This log streaming is then used by a new iOS test runner
script, allowing the display of test suite output at runtime. The iOS test
runner script can be used by any Python project, not just the CPython test
suite.
